This invention relates to a lithium secondary battery using hydric boron carbonitride as the active material of the negative electrode or the positive electrode.
In this specification the term "battery" is used as synonymous with "a single electric cell".
With the rapid progress of electronics and the related technology, remarkable size and weight reductions of electric and electronic devices have been made as well as enhancement of the performance of the devices. Accordingly there is a strong and growing demand for batteries having high energy density and long life as the power sources for the current and future devices.
Among conventional primary batteries lithium battery is notable in respect of high energy density and has further advantages such as good storability with little deterioration and operability over a wide range of temperature. Therefore, there is a deep desire for the development of lithium secondary battery which retains the excellent properties of lithium primary battery.
In most of lithium secondary batteries developed until now metallic lithium or Li-Al alloy is used as the active material of the negative electrode and an oxide such as MnO.sub.2 or V.sub.2 O.sub.5 as the material of the positive electrode. However, a lithium secondary battery of this type cannot bear a large number of charge-discharge cycles, because as the charge and discharge are repeated dendrites of Li precipitate on the Li or Li-Al electrode to cause partial disintegration of that electrode and/or internal short-cirucuiting. To solve this problem it has been proposed to use a compound which makes a reversible intercalation reaction with lithium, such as graphite or linear-graphite hybrid (LHG), as the active material of the negative electrode so that Li may be intercalated in and released from the active material during charging and discharging. Also it has been proposed to use a conductive organic polymer as the active material which incorporates and releases Li. Including these proposals the development of lithium secondary batteries is summarized in Kagaku To Kogyo (Chemistry and Chemical Industry), Vol. 42, No. 9 (1989), pp. 1558-1561, and ibid., pp. 1565-1567.
However, in the case of the negative electrode using graphite the intercalation of Li in the graphite is not smooth because of a low rate of diffusion of Li ion in the interlayer spaces of graphite, and the intercalated Li is not easily released because of strong interaction between graphite and Li. Therefore, it is difficult to realize a desirably high current density. Besides, as charging and discharging are repeated the crystallinity of the graphite reduces whereby the volume of the negative electrode increases.
In the case of using LGH the battery is liable to make internal self-discharge and is relatively small in capacity because LGH is very small in crystallite and hence weak in the power of holding intercalated Li.
In the case of using a conductive organic polymer there is a problem that the negative electrode in the charged state tends to irreversibly react with the electrolyte solution to result in an increase in internal self-dischrage and a decrease in possible charge-discharge cycles. Besides, also in this case the capacity of the battery is relatively low.